There are large reserves of so-called secondary ores containing base and precious metals which it would be desirable to recover. Such secondary ores include oxides, sulphates and partially oxidised or weathered sulphides. The ores may be complex containing portions of sulphide, sulphate, oxide and weathered sulphide, any of which may require a different kind of hydrometallurgical treatment to enable the recovery of metals from the particular one. Nevertheless, whatever the geology, the sulphur content of the ore is much less than would be encountered with a massive sulphide orebody. Accordingly, such ores may be described as "low sulphur ores". Secondary ores of copper including antlerite, brochantite, malachite and other mineral types, typically reflect an association of small amounts of copper sulphide minerals with a large amount of carbonate or oxide minerals. Weathered or partially oxidised sulphides may also be present within the ore. Other base metals exhibit similar oxidised mineralogy, for example lead and zinc.
Generally, such ores may be treated to separate sulphides and the other secondary minerals present. The secondary minerals are not amenable to pyrometallurgical treatment as gangue heating and slag handling requirements are too high. Such costs dictate a hydrometallurgical route to recovery which may involve treatment by a process which involves acid leaching, such being basically the only hydrometallurgical route to recovery. The economics of acid leaching of course will vary with the location of the ore body and proximity to sources of acid producing materials such as sulphide ores, pyrites or sulphur. Nevertheless, in some locations, there will be no source of acid producing material and it may be necessary to plan on the basis that sulphuric or sulphurous acid will require to be imported to the site. The transportation and material costs of pursuing this option may be sufficiently high that the proposed project remains unviable even though on a grade basis the resource would be attractive to develop. The problem may be increased by the acid consuming nature of carbonate minerals that increase acid costs and further impact on economic viability.
Other ores, such as the refractory ores, may include appreciable content of precious metals such as gold and silver. Such ores are also low sulphur ores and as described in U.S. Pat. No. 5,246,486 may have a sulphur content of 0.2 to 0.3%. Thus only a limited extent of bacterial leaching is possible, and such ores need to be subjected to cyanidation or other hydrometallurgical treatment for the liberation of the precious metals.